1. Field of the Invention
The present invention relates to a device and a method to process a color signal input to a display device, and more particularly, to a color signal processing device and method to precisely and simply calculate a maximum color chroma of an input video signal in relation with a hue and a luminance in a color space.
2. Description of the Related Art
Substantial colors refer to color objects that are emitted when a light is permeated through, absorbed into, and reflected by the color objects. The substantial colors are mainly classified into achromatic colors having various degrees of brightness and darkness without a hue, such as white, gray, and black, and chromatic colors having the hue, such as red, blue, and so on. Any color having the hue, even a bit, can be referred to as one of the chromatic colors because the chromatic colors include the hue.
Like the white color is bright, the black color is dark, and the gray color has an average brightness, degrees of brightness or darkness indicate that quantifying a visual intensity of the light reflected by the color object is referred to as the brightness. It is typically considered that a yellow color of a melon is bright, and a purple color of a grape is dark, the brightness exists even in the chromatic colors.
Further, as in the red, the yellow, green, the blue, and the purple colors, the hue is perceived by human beings as a result of the light having diverse wavelengths and illuminating the color objects. For example, wavelengths in a range of 430 to 480 nanometers give a strong feeling of the blue color. Furthermore, the yellow color is felt in a range of 570 to 600 nanometers, and wavelengths of over 610 nanometers are classified as the red color. The achromatic colors such as the black, gray, and white colors each have the color, but not the hue.
Further, differences of high and low color concentrations, that is, degrees of colors not diluted with the white color are referred to as chroma. The chroma indicates a purity of the colors. The colors having a low chroma are washed-out or faded, while the colors having a high chroma are distinct and vivid. The chromatic colors have the chroma, the hue, and the brightness, but the achromatic colors can be referred to as particular colors having the brightness only without the hue and the chroma. As such, the hue, the brightness, and the chroma are referred to as three-color attributes.
Further, a method representing relations among the colors is referred to as a color space or a color model. Different image processing systems have different color models due to different reasons. For example, companies publishing color pictures use a CMY color space. An RGB color space is used for color cathode ray tube (CRT) monitors and computer graphic systems. An HIS color space is employed for systems dealing with the hue, the chroma, and the brightness. Further, the YCbCr color space is used for a JPEG File Interchange Format (JFIF) and the like.
In here, the RGB color space includes the red, green, and blue colors, referred to as three primary colors that can be added with one another. Spectral elements of the colors are additionally combined to obtain resultant colors. As shown in FIG. 1, the RGB color space is represented as a three-dimensional cube having axes representing the red, green, and blue colors, respectively. An origin represents the black color. The white color is at an opposite end of the black color in the cube. The brightness is represented along a line from the black color to the white color. The red color(R) is represented in (255, 0, 0) in 24-bit color graphic systems having 8 bits per color channel, and in (1, 0, 0) in the color cube.
The YCbCr color space is a color space separating light intensity from color information. Y denotes luminance, Cb is a blue color difference signal, and Cr is a red color difference signal. If the RGB is converted into the YCbCr, the Cb has a significant amount of the blue color, and the Cr has a significant amount of the red color. Whereas the RGB signals have the same bandwidth, a CbCr color difference signal of YCbCr signals can be effectively used because a bandwidth of the YCbCr signals is reduced.
The luminance represents the degree of image brightness, and pixel luminance is represented in 8 bits in the ITU-R 601 standard (formerly, CCIR standard). A color difference represents a degree of the image color, and using double 8 bits in the ITU-R 601 standard represents a pixel color. The YCbCr used for MPEG represents a pixel in information of triple 8 bits of luminance Y and color differences Cb and Cr.
Because human eyes are more sensitive to the luminance signal Y compared to the color difference signals Cb and Cr, the Cb and Cr signals are sampled to reduce corresponding data amounts. At this time, a color format not sampled is specified in 4:4:4, a format in case that the color signal is once sampled is specified in 4:2:2, and a format in case sampled once more is specified in 4:2:0. The 4:4:2, 4:2:2, and 4:2:0 formats each refer to a sampling frequency ratio of a luminance signal and two color difference signals. The 4:2:0 format alternately becomes the 4:2:0 format and the 4:2:2 format in odd-numbered lines and even-numbered lines, and one of the formats becomes a representative value, which is specified in the 4:2:0 format. That is, the color information not reduced is referred to as the 4:4:4 format, the color information reduced in half in a traverse direction is referred to as the 4:2:2 format, and the color information reduced in half in both traverse and lengthwise directions is referred to as the 4:2:0 format. Accordingly, the color information becomes one fourth of the luminance information in the 4:2:0 format.
Formula 1 shows one exemplary method converting the RGB into the YCbCr.Y=0.29900R+0.58700G+0.11400BCb=−0.16874R−0.33126G+0.50000BCr=0.50000R−0.41869G−0.08131B  Formula 1
To the contrary, Formula 2 shows an exemplary method converting the YCbCr into the RGB.R=1.00000Y+1.40200CrG=1.00000Y−0.34414Cb−0.71414CrB=1.00000Y+1.177200Cb  Formula 2
FIG. 2 shows a color gamut expressed in the YCbCr color space for the colors represented in the RGB color space according to the method expressed in Formula 1.
The above Formulas 1 and 2 are the International Radio Consultative Committee (CCIR) 601-1 standard, which is a typical method used in the Joint Photographic Experts Group (JPEG). In addition to Formulas 1 and 2, there are various methods to convert the YCbCr into the RGB or vice versa.
Digital components convert the input RGB color signal into the YCbCr color signal for display. Further, the color signal expressed in the YCbCr is processed into the digital signal, and then converted into the RGB signal for display.
However, during converting the RGB color signal into the color signal of YCbCr or the like and then processing the converted signal into a digital signal, conversions made into colors unable to be expressed in the RGB color space may occur. In this case, the processed color signal may be out of the color gamut capable of being displayed.